1. Field of the Invention
The present invention relates to a photomask (reticle) to be used in a photolithography system, and more particularly to a photomask suited to efficiently transfer various fine patterns into different positions on a wafer.
2. Description of the Related Art
The photolithography technology of a reduced projection exposure method has approached the resolution limit with the reduction of design dimensions and the fineness of processing patterns. There has been used a phase shifting method for improving the resolution limit. According to the phase shifting method, the resolution limit is theoretically improved by about twice the resolution limit as compared with a conventional method. The phase shifting method has been described in IEEE Trans. on Electron Devices, Vol. ED-31, No. 6, 1984, pp. 753 to 763, for example (see FIG. 31).
However, the phase shifting method has the following drawbacks. That is, the phase shifting method is difficult to apply to a complex fine pattern such as an actual LSI pattern. More specifically, the phase shifting method is easy to apply to a regular line and space pattern. However, the phase shifting method is hard to apply to an isolated fine pattern, an aperiodic pattern, a plurality of combined patterns and the like.
For the plurality of combined patterns, there has been proposed a multistage phase shifting method described in Extended Abstracts of 51th Autumn Meeting of the Japan Society of Applied Physics (1990), pp. 491 and 492, Lecture No. 27p-ZG-4,5, for example. For the isolated pattern, there has been proposed an auxiliary pattern method disclosed in Japanese Unexamined Patent Publication No. 62(1987)-67514, for example. However, the above-mentioned methods are not easy to apply in practice.
There has also been known a phase shifting method in which a phase shifter 243 is provided only around a transmitting portion (see FIG. 30). This phase shifting method is a so-called self-aligning type (ex. edge enhancement, rim, etc.) and is effective in the resolution of the isolated pattern. In FIG. 30, a self-aligning type of phase shifting reticle sequentially includes a glass substrate 241 as a reticle and the phase shifter 243 from the light source side to the wafer side. The phase shifter 243 is provided through a Cr (chromium) film 242. A method using a self-aligning type phase shifter has been described in Extended Abstracts of 51th Autumn Meeting of the Japan Society of Applied Physics (1990), pp. 492, Lecture No. 27p-ZG-2, for example. Also in the self-aligning type of phase shifter, however, there is utilized the interference effects of light in a light transmitting portion. Consequently, the gradient of a light intensity is lowered, so that the self-aligning type of phase shifter is hard to apply to fine patterns.
When the processing patterns are made finer so as to have almost the same size as an exposure wavelength or less, a quantity of light transmitted through a reticle light transmitting portion is decreased. Consequently, the effects of oblique incident components cannot be ignored for vertical incident components (see FIG. 23). FIG. 24 shows the arrangement of an optical system. In FIG. 24, light emitted from a light source 221 passes through a condenser lens 222. The convergent light is incident on a photomask 21 within the range of an estimated angle .theta.c based on a focal position (for example, a central position represented by a point P2 of an aperture 141 as a light transmitting portion shown in FIG. 14). Then, the convergent light is emitted as transmitted light with an estimated angle .theta.p through a light transmitting portion 223, is projected onto the plane of a projection lens 224 in the direction of an optical axis (in the direction of an arrow A), and is finally projected onto the imaging plane of a wafer. FIG. 16 schematically shows an optical system. In FIG. 16, the reference numeral 225 denotes an imaging plane on the wafer. A waveform B denotes the light intensity of a projected image. In the case where the self-aligning type of waveform expands and the light intensity on the imaging plane is decreased. Consequently, a light intensity contrast (gradient) is lowered so that a resist pattern cannot be resolved. Since the phase difference between adjacent transmitted light is utilized, the phase shifting method is hard to apply to the isolated fine pattern, the aperiodic pattern and the plurality of combined patterns.